Johanblumenberg fnmock

[LironHazan]

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[LironHazan]

@cspotcode I want to merge this PR, wdyt?

[LironHazan]

@johanblumenberg on another topic :) I really liked that implementation, do you think it's enough to be ported or are there any other functionalities in scope of mocking single fns you've added worth grabbing?

[cspotcode]

@LironHazan go for it. I won't have much time for ts-mockito so I probably won't be able to do code review, so I say go for it with whatever you have planned.

[roypeled]

I think that this can be easily achieved with spies, instead of creating a new interface:

const CallbackSpy = spy({ callback: () => 'foobar' });

expect(CallbackSpy.callback()).toHaveBeenCalled()